Fluid couplings have heretofore been used as power transmission couplings for ships, industrial machines and automobiles. The fluid coupling comprises a pump having an annular pump shell and a plurality of impellers radially arranged in the pump shell, and a turbine having an annular turbine shell and a plurality of runners radially arranged in the turbine shell, and is arranged being opposed to the pump, the pump and the turbine being filled with an operation fluid. The thus constituted fluid coupling has the pump that is coupled to a crank shaft (input shaft for the fluid coupling) of, for example, a diesel engine which is a prime mover, and has the turbine that is mounted on an output shaft that is arranged in concentric with the input shaft.
There has further been used a fluid coupling in which the pump shell and the turbine shell are provided with an annular core-ring for orienting the operation fluid.
FIG. 11 illustrates characteristics of a general fluid coupling, wherein the abscissa represents a speed ratio (e) of the pump and the turbine, and the ordinate represents an input capacity coefficient (τ) of the fluid coupling. As will be understood from FIG. 11, the input capacity coefficient (τ) of the fluid coupling becomes a maximum in a state where the speed ratio (e) of the pump and the turbine is zero (0), i.e., in a state where the pump is rotating while the turbine is halting. When the drive unit of a vehicle is equipped with the fluid coupling having the above-mentioned characteristics, a drag torque is inevitably produced in a state where the engine is running with a speed change gear of a transmission being engaged while the vehicle is at rest, i.e., in a state where the input shaft is rotating while the output shaft is halting. The drag torque stands for a transmission torque in a state where the engine is running at an idling speed (e.g., 500 rpm). A large drag torque greatly impairs the stability of idling operation of the engine, and unstable rotation causes the occurrence of abnormal vibration in the drive system. Further, a large drag torque becomes a cause of deteriorating the fuel efficiency during the idling operation.
As a measure for decreasing the drag torque, there has been known a technology for arranging a baffle plate between the pump and the turbine.
A measure for decreasing the drag torque by arranging the baffle plate will now be described with reference to FIGS. 12 and 13. In a fluid coupling shown in FIG. 12, an annular baffle plate BP is mounted on an output shaft OS being arranged between the pump P and the turbine T. In a fluid coupling shown in FIG. 13, on the other hand, an annular baffle plate BP is arranged along the outer circumferential portion of the pump P.
The fluid couplings shown in FIGS. 12 and 13 are using fixed baffle plates which are effective in varying the characteristics of the input capacity coefficient (τ) relative to the speed ratio (e) of the pump and the turbine, but are not capable of varying the τ-characteristics relative to the input rotational speed. That is, when τ (e=0) is lowered to cope with the drag torque, the drag torque during the idling can be decreased to be smaller than that of when there is no baffle plate also causing, however, a decrease in the transmission of torque at the time of start. Namely, the engine must be rotated at an unnecessarily increased speed to start deteriorating, however, the fuel efficiency. On the other hand, when τ (e=0) is increased to increase the transmission of torque at the start, a start torque can be obtained causing, however, an increase in the drag torque during the idling and deteriorating the fuel efficiency during the idling. In the fluid couplings using the fixed baffle plate as described above, there exists a trade-off relationship between the drag torque during the idling and the fuel efficiency, which cannot be solved.
In order to decrease the drag torque, further, JP-A-2001-50309 is proposing a fluid coupling having an annular baffle plate mounted on the inner circumference or on the outer circumference of the core ring of the pump shell or of the core ring of the turbine shell.
When the drive unit of a vehicle is equipped with the fluid coupling, the characteristics thereof should desirably be to lower the transmission of torque during the idling where the speed ratio (e) of the pump and the turbine is zero (0), i.e., when the pump is rotating while the turbine is halting, without sacrificing the transmission of torque at the start of when the engine is rotated at a speed, i.e., when the rotational speed of the pump is high. The fluid coupling disclosed in the above JP-A-2001-50309 is capable of effectively decreasing the transmission of torque or the drag torque during the idling but inevitably results in a decrease in the transmission of torque at the start of when the engine is rotated at a high speed, i.e., when the rotational speed of the pump is high since the baffle plate has been fixed, which, therefore, is not satisfactory yet.